Ink-jet types of image-forming apparatuses are widely used for forming an image on a recording medium by ejection of an ink from ink-ejecting nozzles of a recording head. In such an ink-jet type of image-forming apparatus, a heater element is provided on the inside wall of the respective nozzles, and electric pulses are applied selectively to the heater element to cause bubbling of the ink by film boiling in correspondence with the image to be recorded to eject an ink droplet through the nozzle.
An image can be recorded on a long recording medium sheet (e.g., several meters long) with such an ink-jet type image-forming apparatus. In the recording on a long recording medium sheet, the ink is ejected repeatedly over a long time from the nozzles of the recording head by applying electric pulses repeatedly to the heater elements in the nozzles. Thereby, the thermal energy applied to the ink in the nozzles can not dissipate sufficiently to cause rise of the temperature of the recording head and the ink in the nozzles. This rise of temperature of the ink will increase the amount of the ink droplet (size of the droplet) ejected from the nozzle in one ink ejection. Furthermore, excessive rise of the temperature of the recording head causes decrease of the surface tension of the ink to prevent the ink meniscus formation at the nozzle outlet (ink ejection outlet) to cause defects in the recorded image.
To prevent such a trouble in an ink-jet type image-forming apparatus, the inside temperature of the recording head is detected and the temperature of the recording head is kept below a prescribed temperature by changing the widths of the electric pulses applied to the heater element or by decreasing the delivery speed of the recording medium. (e.g., JP2002-113845A).
When the width of the electric pulse applied to the heater element is changed as mentioned above (when the thermal energy applied to the ink in the nozzle is changed), the amount of the ink droplet ejected from the nozzle is changed during the image formation on one sheet of the recording medium to cause irregularity of the image density on the one recording medium sheet. On the other hand, when the width of the electric pulse is kept unchanged to avoid the above irregularity of the image density, the heater element is overheated to cause adverse effect on the life of the heater element.